1. Field of the Invention
The present invention relates to a photocatalyst which promotes the oxidation-reduction reaction of materials included in a solution or a gas by using photoenergy, and also relates to a manufacturing method therefor.
2. Description of the Related Art
When semiconductors, such as titanium oxide, strontium titanate, or cadmium sulfide, are irradiated by light, such as ultraviolet radiation, having higher energies than in the respective semiconductor band gaps, electrons accumulate in conduction bands, and holes are generated in valence bands.
Photocatalyst activation, in which organic compounds in the atmosphere are decomposed by oxidation-reduction reactions based on electron transfer caused by the accumulated electrons and the holes, and thereby providing sterilizing effects, is known and studies are being undertaken for industrial application of this photocatalyst activation.
Since the photocatalytic reaction is activated at the surface of the photocatalyst, it is necessary to increase the specific surface area of the photosemiconductor in order to enlarge the surface of the photocatalyst, for the purpose of effectively using the function of the photocatalyst.
Japanese Unexamined Patent Application, First Publication No. Hei 11-144772 discloses a semiconductor electrode consisting of a porous metallic oxide, in which two pore groups are interspersed, one group consisting mainly of large pores and the other group consisting mainly of small pores.
In this semiconductor electrode, the specific surface area of the photocatalyst is enlarged by the small pores, and functional molecules, such as sensitizing dyes, can be delivered throughout the porous metallic oxide by the large pores. Therefore, the conversion efficiency of a photoelectric cell, for example, can be improved.
However, according to the Japanese Unexamined Patent Application, First Publication No. Hei 11-144772, a set of steps of applying a colloidal solution onto a support by using the dipcoat method, the spincoat method, the doctorblade method, or the like, and of baking it at a high temperature of approximately 400xc2x0 C. is required to be repeated. Therefore, the following problems occur:
(1) Since the coated film thickness, which can obtained by one set of the processes of applying and baking by the dipcoat method, the spincoat method, the doctor blade method, or the like, is thin, the set of processes must be repeated many times until the coated film acquires the desired thickness, so it takes a long time to obtain the desired thickness.
(2) Once a thick film is formed by the above-stated application process, fine cracks arise due to contraction during film formation, the adhesion of the film to the support or the binding of the films to each other is lowered, so the films tend to separate from each other or from the support.
(3) Since it is required to separate the step of applying a colloidal solution onto the support from the step of baking, to carry out the step of baking after the step of applying, and to repeat the set of the steps of applying and baking until obtaining the desired film thickness, the manufacturing process is complicated.
The present invention has been made to solve the problems described above. An object of this present invention is to provide a photooxidation catalyst and its manufacturing method, which increases the adhesion of the photooxidation catalyst to the support, and for which the steps of applying and baking in its manufacturing process are simplified.
The present invention provides a photooxidation catalyst comprising a support and a metallic oxide layer provided on the support, wherein the metallic oxide layer has a cross-sectional structure in which a surface region of the metallic oxide layer is more porous than a base region of the metallic oxide layer which is adjacent to the support.
The metallic oxide layer may be formed by applying a colloidal solution including at least one metallic oxide precursor on the support while baking the colloidal solution applied on the support. The metallic oxide layer may have a two-layered structure, in which a low density metallic oxide layer of a porous structure is laminated onto a high density metallic oxide layer. The two-layered structure may be formed by applying a first colloidal solution which includes at least one metallic oxide precursor while baking the first colloidal solution applied onto the support, so as to form the high density metallic oxide layer, and thereon applying a second colloidal solution which includes at least one metallic oxide precursor and an organic compound while baking the second colloidal solution applied onto the high density metallic oxide layer, so as to form the low density metallic oxide layer.
The present invention also provides a manufacturing method of a photooxidation catalyst, comprising the steps of producing a gel from a mixed solution including at least one metallic compound, hydrolyzing and dehydrocondensing the metallic compound in the gel to produce a colloidal solution including a metallic oxide precursor, and applying the colloidal solution onto a support while baking the colloidal solution applied onto the support to form a metallic oxide layer.
The metallic oxide precursor may include fine particles of metallic oxide and/or partially hydrolyzed metallic compounds. The step of applying the colloidal solution onto a support while baking the colloidal solution applied on the support may be carried out by spraying the colloidal solution on a preheated support, and approximately simultaneously baking the colloidal solution by heat of the preheated support to form the metallic oxide layer. The step of applying the colloidal solution onto a support while baking the colloidal solution applied onto the support may be carried out to form the metallic oxide layer by the steps of applying a first colloidal solution which includes at least one metallic oxide precursor while baking the first colloidal solution applied onto the support, so as to form a high density metallic oxide layer, and subsequently applying a second colloidal solution which includes at least one metallic oxide precursor and an organic compound while baking the second colloidal solution applied onto the high density metallic oxide layer, so as to form a low density metallic oxide layer of the porous material.
The manufacturing method of a photooxidation catalyst may further include a step of impregnating the metallic oxide layer formed on the support with a metallic compound solution and then baking.
The present invention also provides a manufacturing method of a photooxidation catalyst comprising the steps of heating a support, applying a first colloidal solution including at least one metallic oxide precursor and a solvent onto the support while heating the support, and removing the solvent from the first colloidal solution to form a first metallic oxide layer on the support.
The manufacturing method may further include the steps of applying a second colloidal solution including at least one metallic oxide precursor, an organic compound, and a solvent onto the first metallic oxide layer, and removing the solvent from the second colloidal solution to form a second metallic oxide layer on the first metallic oxide layer which is less porous than the second metallic oxide layer.